In recent years, since problems of global warming, which has occurred due to carbon dioxide emission by use of fossil fuels and radioactive waste and contamination by nuclear power plant accidents have become serious, interest in the global environment and energy production are growing. Under these circumstances, solar power generation that uses solar light as an inexhaustible and clean energy source, geothermal power generation using geothermal energy, wind power generation using wind power, and the like have been put into practice all over the world.
Solar power generation using solar batteries employs various forms corresponding to output scales ranging from several watts (W) to several thousand kilowatts (kW). A representative system using a solar battery is a solar power generation system which collects a direct current power generated by a solar battery array including a plurality of solar battery modules connected in series and in parallel. The direct current power is converted (DC-AC-converted) into an alternating current power by a power converter such as an inverter and supplied to the loads of subscriber's houses or a commercial alternating current power grid (to also be simply referred to as a “power grid” hereinafter).
An inverter used in such a solar power generation system normally has a capacity of 3 to 5 kW and is mainly attached to the outer wall or entrance of a house. Assume that the capacity of the solar battery array is large, or a plurality of solar battery arrays are installed at different locations. For example, assume that a 3-kW solar battery array is installed on the south side of the roof of a house, and another solar battery array having the same capacity is installed on the north side. In such a case, a plurality of inverters connected in parallel are used.
In recent years, inverter-integrated solar battery modules (also called AC modules) have been developed in which each solar battery module has an inverter with a corresponding capacity (about 100 W) and outputs an alternating current power. In the future, a solar power generation system is expected to be built by using a solar battery array which includes several tens of AC modules connected in parallel. Even in this arrangement, the inverters are connected in parallel.
Generally, in the system coordination operation executed by connecting a power generation system such as a solar power generation system using natural energy to an existing power system, when a power outage occurs in the power system due to an accident or the like, any islanding operation (islanding operation state) of the inverter must be avoided. For this purpose, this state must quickly be detected on the power generation system side, and the operation of the power generation system must be stopped.
To do this, two schemata, i.e., a passive schema and active schema can be used. In the passive schema, the electrical parameters of the system are monitored in the inverter, and the islanding operation state is detected on the basis of a change in parameter values in case of power outage. In the active schema, a variation (disturbance signal) is always given to the voltage or frequency of the output from the inverter, and the islanding operation state is detected by detecting an inverter output variation which is conspicuous in case of power outage.
However, when a number of inverters having the same active detection schema are connected in parallel and execute the system coordination operation, the disturbance signals cause mutual interference. For this reason, the islanding operation detection sensitivity may decrease, and it may become impossible to detect the islanding operation state.
To solve this problem, in Japanese Patent Laid-Open No. 2000-270482, a master/slave selector switch is arranged in each of power conditioners connected in parallel. In the example described in this reference, this switch is operated upon installing the inverters to determine whether to use each inverter as a master machine or slave machine. The islanding operation detection schema is executed by only the master machine. With this arrangement, any decrease in islanding operation detection sensitivity is prevented.
However, the above-described method has the following problems.
When the master machine is set by the switch in advance, and a failure occurs in the master machine, the inverter set as the master machine is not activated so the control to be executed by the master machine cannot be executed. That is, the islanding operation detection function does not operate, and safety suffers.
In the system using AC modules in which one inverter is connected for each solar battery module, when a solar battery module malfunctions or is shaded, the inverter is often not activated, although this problem is rarely posed in a system in which one inverter is connected to every capacity (3 to 5 kW) of the solar battery.
When a number of inverters are connected in parallel, as in the system using AC modules, even when a failure occurs in one inverter, the failure can hardly be recognized. If a failure occurs in the master machine, the operation is continued without executing islanding operation detection for a long time.
Such problems are not limited to solar power generation systems and are common to power generation systems using any other direct current power sources.